1. Field of the Invention
The present invention provides a method and related apparatus for image processing, and in particular, a method and related apparatus for clipping a three-dimensional object according to a viewing range and recording vertices after clipping in a better primitive, and hence improving image processing performance.
2. Description of the Prior Art
Presenting various documents, information, results and data with graphics makes browsing, viewing, observing and arranging easy for users. This has also become one of the most important functions of a modern computer system. Virtual reality graphical presentation allows the users to view a three-dimensional (3D) object, which plays an important role in fields of medical images, Computer-Aided Design/Computer-Aided Manufacture (CAD/CAM), virtual reality (such as flight simulation), multimedia games, etc. Thus, virtual reality 3D graphical presentation technologies are one research emphasis of a modern information industry.
In techniques of presenting 3D graphics, 3D objects and scenes in a virtual 3D space are processed to have the objects and scenes projected and displayed on a two-dimensional (2D) screen. The 2D screen can be equivalently seen as a photo of the 3D space which is taken by a virtual camera. Viewing graphics displayed on the 2D screen is just like observing the objects and scenes through a 2D image (i.e. a photograph) taken by a virtual camera. And in this way, the prior art can realize graphical representations of the virtual reality.
In modern techniques of presenting 3D space, a 3D object in a virtual 3D space can be formed with many faces, so the 3D object is also called a polygonal object. In fact, due to a special geometric characteristic of a triangle, i.e. that three vertices are always coplanar, faces of a 3D object are divided into one or more triangular planes with a triangle as a primitive. Finally the 3D object takes the triangular plane as a primary structural unit; the 3D object is formed by assembling all of the triangular planes. Since a location of a triangular plane can be defined by coordinates of its three vertices, a location and volume of an object occupying a 3D space can be represented with a vertex list that compiles the coordinates of each triangular plane of an object.
For people who are familiar with the art, when forming planes of any shape or 3D objects with combinations of triangular planes, since different triangular planes typically align together to form particular primitives, when recording the vertices of the triangular planes from the combinations of the triangular planes in the vertex list, the vertices can be recorded in different primitives according to different modes and sequences in a same list. Please refer to FIG. 1. FIG. 1 represents how to record vertices of a polygon in a vertex list with different primitives. FIG. 1 shows a pentagon as an example. The pentagon has four vertices A1 to A5, which can be divided into three triangular planes as shown. When describing the pentagon with vertices of the triangular planes, the three triangular planes can be seen as three independent triangles. Therefore, according to a triangle-list primitive, coordinates of the vertices are recorded in a vertex list as {A1, A2, A3, A1, A3, A4, A1, A4, A5}, which is simply recording the vertices of each triangle sequentially. On the other hand, the three triangular planes can be seen as a triangle-fan primitive since the three triangular planes share a vertex. The triangle-fan has a vertex A1 as its center, and therefore a sequence {A1, A2, A3, A4, A5} is recorded in a vertex list according to the triangle-fan primitive. The triangle-fan primitive takes the vertex A1 as the center of the fan and records each vertex consecutively. Other vertex list primitives include a triangle-stripe, etc.
It can be seen from the above discussion of primitives that given one triangle plane configuration, the vertex list can have different contents if the coordinates of the vertices of the triangles are recorded in the vertex list according to the different primitives. Thus, the vertex list must have a corresponding primitive. When reading the vertex list, the vertex list must be read with respect to a sequence defined by a corresponding primitive, so that a correct description of a polygon or an object can be obtained from the vertex list.
By describing objects and scenes in a virtual 3D space with vertex lists, and appropriately processing each vertex list, a position of each vertex as it is projected on a 2D screen can be calculated. Effectively projecting the 3D space on the 2D screen is equivalent to forming an image on the 2D screen by a virtual camera.
However, like a real camera, the virtual camera also has a specific viewing range. For example, an object placed right in front of the camera is projected in the center of a photo (a 2D screen), while an object placed at the left/right side of the camera may not be captured by the camera and hence may not be presented in the photo. An object that extends from the front of the camera to the left/right side of the camera is only presented partially in the photo; the rest of the object is outside the viewing range and hence not shown in the photo.
Similarly, in 3D graphics presentation techniques, when projecting a 3D object onto a 2D screen, it is necessary to calculate whether the object lies in a range of a virtual camera. If an object lies completely outside the viewing range, that means the object would not be projected in a photo (2D screen), and that further means there is no need to process a redundant calculation for the object outside the viewing range during image processing. Correspondingly, if an object lies partially inside the viewing range and partially outside the viewing range, a clipping procedure is executed on the object to keep a portion of the object that is captured inside the viewing range and only project the portion kept onto the 2D screen.
Clipping an object in a 3D space according to a viewing range involves clipping triangular planes since the object is constructed with triangular planes. If the object extends across the viewing range, then one or more triangular planes will intersect with boundaries of the viewing range, meaning these triangular planes comprise some vertices inside the viewing range and some vertices outside the viewing range. For instance, if a triangular plane comprises a vertex outside of the viewing range, after clipping, the triangular plane is clipped to form a clipped plane that might be a quadrangle or a pentagon. By replacing triangular planes of the object that extends across the viewing range with clipped planes, and discarding triangular planes that lie completely outside the viewing range, triangular planes that are originally inside the viewing range are kept, and hence a clipping process of the object is completed.
As discussed above, to project an object in a 3D space onto a 2D screen, the object should be constructed with triangular planes. So when processing clipping as described above, clipped planes formed after clipping still need to be divided into triangular planes, and further, vertices of the triangular planes need to be recorded. For example, if a triangular plane is clipped to form a quadrangle clipped plane, the clipped plane will be divided further into two triangular planes. Therefore, the clipped plane is represented by the two triangular planes.
In the image processing techniques of the prior art, when recording the vertices of the triangular planes of each clipped plane, the vertices of each clipped plane are recorded in a vertex list as a triangle-fan primitive. In other words, if an object comprises N triangular planes crossing in and out of the viewing range, after clipping the N triangular planes into N clipped planes, the prior art records the vertices of each clipped plane corresponding to N triangle-fan primitives in vertex lists.
After completing the clipping procedure, the 3D graphics presentation technique can perform pixel-level processing to calculate an image that will be presented on the 2D screen by each triangular plane of the object. Since the 2D screen may present images with pixels as units, when performing the pixel-level processing, numbers of pixels occupied by each triangular plane can be calculated, along with a color, brightness, and texture thereof.
During practical pixel-level processing, the process receives vertex lists corresponding to each object, reads/connects corresponding triangular planes according to a primitive corresponding to each vertex list, and perform relative calculations for the 2D screen according to positions of the triangular planes. However, because the prior art uses a plurality of vertex lists to describe a plurality of clipped planes, processing efficiency is affected during pixel-level processing.
As mentioned above, when reading a vertex list, a reading order must be set according to a primitive corresponding to the vertex list first, and so as to an original plane can be correctly formed by connecting vertices. However, the process of setting the reading order consumes a large amount of system resources. In particular, because the prior art generates a plurality of vertex lists for a plurality of clipped planes when clipping an object, the result is that the setting and reading processes are repeated many times for each vertex list, which increases the system resources consumed and affects processing efficiency.
For example, a modern computer system usually uses graphics processing hardware to realize pixel-level processing, wherein pipeline processing hardware is used to read vertex lists. Since the prior art records vertices of different clipped planes in many vertex lists when processing the vertices of the clipped planes, the pipeline processing hardware must repeatedly flush the pipeline when reading each vertex list before it can perform setting of the reading order, which brings down the overall graphics processing performance.